The purpose of our current research is to investigate the hypothesis that emotional and locomotor behavior as displayed in aggression can be modified by the simple expedient of regulating the balance in the diet of the amino acid precursors of catecholamines and indoleamines. A highly reliable test of territorial aggression in CF-1 mice developed in our laboratory was used in these studies. Our results have demonstrated beyond question that mice fed semi-synthetic diets containing 12 per cent casein protein supplemented with tyrosine are far more aggressive than those fed unsupplemented diets. A similar, but less pronounced effect was obtained using phenylalanine supplements. On the other hand, tryptophan resulted in a variable effect, depending on the level of supplementation used. The results raise a number of questions which cannot be answered by any simple concept of the relationships between catecholamine or indoleamine neurotransmitters, their precursor amino acids, and behavior. This requires further investigation into biochemical mechanisms. Thus, we propose in this renewal application to analyze the metabolism, uptake, receptor activity, etc., of these neurotransmitters to determine the effect of dietary amino acid supplementation on them, at the same time monitoring behavioral correlates. A number of pharmacological agents having well-documented actions on catecholaminergic and serotonergic systems will be used in conjunction with the amino acid supplements to manipulate these mechanisms. The possible significance of such an investigation would be to provide in part a scientific basis for dietary control as a valuable adjunct to psychiatric treatment and the modification of human behavioral patterns.